Projection displays have numerous home and commercial applications. For example, rear-projection television displays are gaining market share for home use. Displays have also been important as information-conveying devices, such as those used in aircraft to provide terrain or flight information data. Ambient light may negatively impact images provided by the displays by decreasing contrast which generally results in a less ear image. The impact of ambient light is more significant in display applications where acquisition of information from the display is particularly important. For example, avionics displays provide a visual indication of in-bound terrain, aircraft status, or other flight information particularly for real-time decision-making, and ambient light may diminish the visual presentation of avionics displays.
Rear projection displays generally have a reduced contrast when used with bright ambient light. This is generally due to the backscatter of a portion of the ambient light by rear projection screens. One common approach to offset the impact of ambient light on displays is to increase the brightness of the projection display which generally increases the brightness of the image while the brightness of the backscattered light remains unchanged. This approach generally increases a contrast ratio of the image where the contrast ratio is a ratio of the highest possible luminance of the image to the lowest possible luminance of the image. The luminance includes any contributions from ambient light. In general, this approach is typically limited by the available brightness of the display. For applications where the display is to be viewed in bright sunlight, it is often difficult or not practical to build a display that is sufficiently bright.
Another approach is to use a neutral density filter in front of the display screen. The neutral density filter is generally not wavelength specific and typically attenuates light across the visual spectrum. Ambient light passes through the filter when traveling towards the screen and passes through the filter when reflected from the screen towards a viewer. The net effect of this neutral density filter is to attenuate the ambient light more than the light from image. When this is done, the contrast ratio of the display is improved, although generally at an overall loss of luminance. A further approach is to use a circular polarizer to improve contrast over the full spectral range of the display. This approach can be effective for many surfaces that reflect with appropriate polarization properties but may also have a significant impact on light throughput from the projector.
Another approach to improving the contrast ratio of the display is to use out-of-band contrast enhancement filters. These filters have been used in the past with displays that have narrow, in-band wavelength regions. The spectral regions that contain the desired image data at the correct wavelength are the in-band wavelength regions. These in-band regions typically correspond to independently modulated colors that are used to generate a color gamut of the display. These colors are referred to as the primary colors for the display, but more than one in-band region may be in a given primary color. Often these primary colors correspond to red, green, and blue. It is also possible to have two or more than three primary colors that do not include red, green, or blue. The spectral regions that do not contain significant amounts of the desired light are referred to as the out-of-band wavelength regions. Light that is present in these out-of-band regions tends to desaturate the primary colors. Removing light from the out-of-band wavelengths, for example in the yellow regions of the visible spectrum, will often increase the saturation of the primary colors and increase the color gamut of the display.
Cathode ray tubes (CRTs) are one example of a display with well defined in-band and out-of-band regions. Because these displays have significantly wide out-of-band regions, selectively absorbing such bands does not generally result in a significant display brightness penalty but does provide a contrast ratio improvement. For example, if a filter absorbs all of the out-of-band light between the red and green primary colors, the red and green primaries are not changed, but the ambient light is reduced. When implementing this approach, substantial efforts are typically made to minimize any absorption of the in-band wavelengths while maximizing the absorption in the out-of-band regions. Rare earth glasses have been used extensively for creating out-of-band absorbing contrast enhancement filters.
Further complicating projection display performance, limited types of light sources are available for projection displays. Such light sources are generally not sufficiently spectrally balanced, particularly for avionics displays, and are classified a having fixed spectral output. A fixed spectral output refers to a lack of independent adjustment at the source of the relative amount of light in different color bands. The inherent spectral distribution associated with these sources may possibly be changed but generally can not be changed after the source is manufactured. Typically after the source is manufactured, change to the spectral distribution is accomplished by altering the total light output otherwise the range of achievable change is substantially small. Arc lamps and single die white light emitting diodes (LEDs) are example of these types of sources. Short arc, high intensity discharge lamps, such as Mercury and Xenon lamps, are commonly used in projection displays, and these lamps generally have a fixed spectral distribution. For many applications, the chromaticity limitations of the displays may be tolerated. However, where a specific color requirement is needed for the display, these types of lamps may not be suitable. For example, Mercury lamps tend to be deficient in red light while providing ample blue and green light.
One approach to this problem is to configure the projection display to be more efficient for the deficient wavelength band. For example, for a red light deficient light source, the pathway for red light may be separated and optimized independently from the pathway for a cyan light. In this example, the efficiency of the cyan path is deliberately decreased with respect to the red path in order to obtain the desired color requirement. Implementation of this approach may be made through adjusting amplitudes of the deficient light bands to compensate for the variations such as by adjusting beam currents in a CRT. These reductions in efficiency due to chromaticity adjustment and ambient contrast enhancement may be problematic in applications such as avionics displays, where more stringent requirements are often placed on luminance, power, and other related requirements as well.
Accordingly, it is desirable to provide high ambient contrast enhancement of projection displays. In addition, it is desirable to provide color correction and contrast enhancement of projection displays having light sources providing a fixed spectral output, and to accomplish this with minimal impact to the optical efficiency. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.